This invention relates to a system for using conventional telephones as part of an intercommunication system. In particular, this invention relates to an inexpensive and simple apparatus for using extension telephones as an intercom system.
Intercom systems have long been regarded as useful additions to a telephone system. An example of a patent disclosing such an intercom system is Jetzt et al., U.S. Pat. No. 3,725,601, entitled "Direct Station Selection Private Intercom Exchange System." Jetzt et al. disclose a modified telephone handset that provides some of the features of a private branch exchange (PBX). One of the features provided by Jetzt et al. is the provision of intercom lines, with one intercom line plus a single line for each telephone that is adapted for interconnection to other telephones in the intercom system. In the system taught by Jetzt et al., for L outside lines and N inside handsets, each handset must be supplied by a cable containing (5L+3N+4) conductors. Thus, in the simplest system employing one outside line and N internal extensions, Jetzt et al., would require (3N+9) conductors. This represents a disadvantage that is overcome by the present invention.
Magnusson, U.S. Pat. No. 4,060,700, entitled "Two-Party Telephone System," discloses a two-party telephone system that features external calls to and from each of the two telephone sets of the system and also a speech connection between the telephone sets. Magnusson uses a dedicated wire pair for the speech connection between the telephone sets. This extra wire pair represents a disadvantage that is overcome by the present invention.
The same distinctions apply to U.S. Pat. Nos. 4,538,031 and 4,459,434, which are both issued to Benning et al. These patents teach an intercom system that has dedicated wires, a disadvantage that is overcome by the invention.
Lee, U.S. Pat. No. 4,636,589, entitled "In-Line Telephone Intercome System," teaches an intercom system that uses two or more telephones on the same line as the intercom transmitters and receivers. However, it is not apparent from an inspection of this patent how the intercom system can work in the case of an incoming call or how the intercom system can overcome the dial tone that results when one or the other of the telephones is picked up. A properly functioning intercom system that uses the telephone lines should hold an incoming call when the intercom system is in use and it should permit the intercom users to pick up the incoming call.